This invention relates to a photochemical reactor element containing microcapsulated TiO.sub.2 photocatalyst.
Titanium dioxide has long been used as a white pigment. It is known that intact TiO.sub.2 crystals or particles have a strong photocatalytic activity and, therefore, may cause photochemical degradation of paint films known as "chalking" when the TiO.sub.2 -containing films are exposed to sunshine in the presence of oxygen and water. Consequently, almost all commercial TiO.sub.2 pigments have been coated with one or more dense layers of SiO.sub.2, Al.sub.2 O.sub.3, ZrO.sub.2 or other metal oxides to retard the photochemical degradation of the paint film.
The TiO.sub.2 photocatalyst finds unique use, instead, in photochemically decomposing or destroying malodorous substances, air pollutants or microorganisms. TiO.sub.2 photocatalyst may also be used in producing superhydrophilic surfaces on a substrate which are hardly susceptible to soiling. See, WO96/29375 published Sep. 26, 1996. In these applications, the TiO.sub.2 photocatalyst occurring as powder not only needs to be immobilized on a substrate but also needs to be exposed at least in part to the atmosphere and light. It is also necessary for a matrix or binder used to immobilize the TiO.sub.2 particles to be hardly susceptible to TiO.sub.2 catalyzed photochemical reactions.
Several attempts have been made to meet these conditions. JP-A-05253544 discloses a method of forming a TiO.sub.2 photocatalyst layer on a ceramic substrate such as building tiles by applying titania sol onto the glaze layer of the tile before melting the glaze layer, and then firing both layers. WO96/29375 discloses a similar method comprising applying a mixture of TiO.sub.2 sol and SiO.sub.2 sol onto the surface of a heat resistant substrate to form a film and then firing the film. This method is used to form a superhydrophilic coating on ceramic or glass substrates such as building tiles, mirrors or glass lenses. The above two methods suffer from disadvantages that the substrate is limited to ceramics, glass or other fire-resistant materials and that the photocatalyst layer is frangible and susceptible to abrasive wear. JP-A-08067835 and JP-A-08141503 disclose the use of an inorganic binder material to immobilize the photocatalyst particles, while WO96/29375, discloses silicone polymers as a binder or matrix of TiO.sub.2 photocatalyst particles to form photoreactive films on a substrate. Although films formed of silica or silicone polymers are less susceptible to the photochemical degradation than organic binder resins conventionally used in the paint industry, only a portion of the photocatalyst particles that are exposed to the atmosphere can participate in the photochemical reaction and the remainder of particles that have been fully embedded in the matrix body are not available to the photochemical reaction.
JP-A-08131842 discloses a photochemically active coating film formed of a thermoplastic or thermosetting resin. To expose the photocatalyst particles adjacent to the surface, the film is irradiated with UV light to photochemically decompose the matrix resin surrounding the photocatalyst particles. Obviously, this treatment is intentional "chalking" which remarkably impairs the film integrity.
JP-A09031335 and JP-A-10005598 disclose TiO.sub.2 photocatalyst particles having a porous coating of photochemically inert inorganic materials. The porous coating layer prevents direct contact of the photocatalyst particles with the surrounding matrix and, therefore, may decrease the photochemical degradation of matrix. This treatment compromises the photocatalytic activity of TiO.sub.2 particles as in the case of conventional TiO.sub.2 pigments. However, the photochemical degradation of matrix is not fully avoidable when used at a concentration sufficient to exhibit a desired photocatalytic activity.
All of the above-discussed prior art commonly suffer from disadvantages associated with the fact that only a portion of the photocatalyst particles that are exposed to the atmosphere can participate in the photochemical reaction and the reminder is not available to the photochemical reaction.
A need exists, therefore, for a TiO.sub.2 photocatalyst composition and a system which may elimate or ameliorate various problems associated with the above-discussed prior art.